With specific clinical examinations or treatments there is the need to register 2D projection images, especially x-ray fluoroscopy images, available in real time during the investigation, relative to a previously recorded 3D image data record, to enable the real time flouroscopy images to be overlaid with anatomical image information from the 3D image data record. Examples of such applications are for example minimally invasive investigations or treatments with endoscopes, laparoscopes or catheters, which are introduced into the patient's body via small openings in the body in each case. Catheters are frequently used within the framework of cardiological examinations, for example with arrhythmias of the heart, which are currently treated by what are known as ablation procedures. In such cases a catheter is introduced during the recording of real time x-ray images, so-called fluoroscopy images, via veins or arteries into a heart chamber. In the heart chamber the tissue causing the arrhythmia is ablated by the application of high-frequency current, i.e. left behind as necrotic tissue.
The medical/technical difficulty with such interventions lies in that fact that, although the catheter can be visualized very exactly and at high resolution in the fluoroscopy images during the x-ray checking, the anatomy of the patient is only shown insufficiently in the fluoroscopy images.
In accordance with the method known from US 2003/0220555 A1 it is thus proposed to use a 3D image data record of the investigation area recorded pre-operatively to register the intra-operative 2D fluoroscopy images relative to the 3D image data record, to determine the spatial position and orientation of the catheter tip with reference to the 2D fluoroscopy images and to use this information to include the catheter tip in a presentation of the 3D image data record. The spatial position and orientation of the catheter tip is determined here by back projection from at least two 2D x-ray images, which is relatively simple in this case, since the form and size of the catheter tip is known in advance and this can be recognized very clearly and at high resolution on the fluoroscopy images
The precisely-positioned inclusion of the catheter tip in the 3D data record however requires that the 3D image data record as well as the two 2D fluoroscopy images are registered with each other, i.e., that their coordinate systems are correlated to each other via a transformation matrix. To perform this so-called 2D-3D registration various methods known from the prior art are named in the US 2003/0220555 A1. With image-based registration for example an “artificial” projection image is calculated in each case iteratively from the 3D image record and compared to the 2D image fluoroscopy record actually obtained. This process is repeated while varying the angle of projection until such time as a sufficient match between the artificial projection image, the so-called “digitally reconstructed radiogram (DRR)”, and the true 2D fluoroscopy image is obtained.
Another 2D-3D registration algorithm uses a landmark-based registration: To this end special anatomical features such as for example the heart surface or specific vessel branching points etc. are used, which are recognizable both in the fluoscopic images and also in the 3D image data record. Further 2D-3D registration algorithms are described are in the Article of J. Weese, T. M. Buzug, G. P. Penney and P. Desmedt “2D/3D Registration and Motion Tracking for Surgical Interventions”, Philips J. Res. 51 (1998), pages 299 to 316. In this method too so-called pseudo-projections are calculated from the 3D image data and compared to the x-ray projection images.
These 2D-3D registration algorithms are time-consuming and relatively unstable as a result of their iterative “trial and error” character. This applies especially if the pre-operative 3D image data record has been generated with a different modality to the fluoroscopy images, that is with MR for example, so that the artificially calculated projection image does not necessarily match the measured fluoroscopy image.
DE 102 01 644 A1 discloses a method for registering an intraoperatively recorded 3D image data record with the patient coordinate system, with the 3D image data record having been calculated from a series of projection images recorded for example with a C-arm device. Marker points attached to the patient which are arranged at least partly outside the reconstructable 3D volume are used for the registration. The marker points are however recorded in at least two 2D projection images, on which the 3D image is calculated, and their spatial position is computed with the aid of the known projection geometry. These are related to the marker coordinates in the patient coordinate system. This allows a registration of the 3D image volume to the patient coordinate system. The method requires however, that the 2D projection images used are already registered to the 3D image volume.